Clinical studies indicate that some diabetic patients without retinopathy display an abnormality of the blood-retinal and blood-aqueous barriers to the angiographic tracer dye, fluorescein. Early dysfunction of the blood-ocular barriers could influence the development of vascular disease in the diabetic eye. Diabetic rats also demonstrate abnormal accumulation of fluorescein in vitreous and aqueour humors, similar to that seen in diabetic humans. In addition, they show a long lasting increase in plasma membrane surface area at the basal aspect of the retinal pigment epithelium, a structural change that may be linked to altered permeability of this barrier layer. We propose to continue studies directed toward indentification of the nature and site of the functional and structural abnormalities of the blood-ocular barrier in diabetic rats. Electron microscopic morphometry, quantitative fluorescence microscopy, and biomicroscopic fluorophotometry will be utilized in experiments designed to achieve the following specific objectives: 1) To characterize early structural changes of the blood-ocular barrier layers with particular attention to measurement of cell membrane area at specific surfaces of these highly polarized cells. 2) To continue study of fluorescent dye distribution across the blood-retinal barrier after intravenous and intravitreal injection using conditions that will allow definition of tracer influx and efflux through the pigment epithelium and vascular endothelium. 3) To establish the cause of increased fluorescein concentration in aqueous humor to determine whether this abnormality reflects a dysfunction common to all components of the blood-ocular barriers and to evaluate its influence on fluorescein concentration in vitreous humor and retina. 4) To explain, as a necessary adjunct of intraocular dye distribution studies, the known abnormalities of fluorescein distribution in the plasma. The experimental results are expected to advance understanding of the diabetes-related abnormality in the rat and, of equal importance, will provide a necessary base of knowledge to support study of this abnormality in diabetic humans.